Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004head....8.2004t&link_type=abstract
American Astronomical Society, HEAD meeting #8, #20.04; Bulletin of the American Astronomical Society, Vol. 36, p.945
Astronomy and Astrophysics
Astronomy
Scientific paper
Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources, in low hard state, steep power-law (soft) state and in transition between these states. Strong QPOs (> 20% rms) are present in the power density spectrum in the spectral range where the power-law component is dominant ( i.e. 60-90 %). This evidence contradicts the dominant long standing interpretation of QPOs as a signature of the thermal accretion disk. We present the data from the literature and our own data to illustrate the dominance of power-law index-QPO frequency correlations. We provide a model, that identifies and explains the origin of the QPOs and how they are imprinted on the properties of power-law flux component. We argue the existence of a bounded compact coronal region which is a natural consequence of the adjustment of Keplerian disk flow to the innermost sub-Keplerian boundary conditions near the central object and that ultimately leads to the formation of a transition layer (TL) between the adjustment radius and the innermost boundary. The model predicts two phases or states dictated by the photon upscattering produced in the TL: (1) hard state, in which the TL is optically thin and very hot (kT ˜ 50 keV) producing photon upscattering via thermal Componization; the photon spectrum index Γ ˜ 1.7 for this state is dictated by gravitational energy release and Compton cooling in an optically thin shock near the adjustment radius; (2) a soft state which is optically thick and relatively cold ( kT ˜ 5 keV); the index for this state, Γ ˜ 2.8 is determined by soft-photon upscattering and photon trapping in converging flow into BH. In the TL model for corona the QPO frequency ν high is related to the gravitational (close to Keplerian) frequency ν {K} at the outer (adjustment) radius and ν low is related to the TL's normal mode (magnetoacoustic) oscillation frequency ν MA. The observed correlations between index and low and high QPO frequencies are readily explained in terms of this model. We also suggest a new method for evaluation of the BH mass using the index-frequency correlation.
Fiorito Ralph
Titarchuk Lev
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